Many types of cylinder head gaskets are known. These fall mainly into two categories, namely, fibrous gaskets and metallic gaskets, or more precisely metallic-elastomeric gaskets.
Fibrous gaskets, which make up the main part of the market, comprise a metallic reinforcement, more particularly a metallic sheet provided with lugs, a packing based on a cardboard of compressible fibrous material which is hooked onto the lugs of the sheet, and strengthening elements of different types, in particular localized deformations and/or eyelets and/or inserts.
Such gaskets present the important advantage, of permitting good distribution of the clamping loads, whereby it is possible to reduce the difference in stiffness between the strengthening elements and the rest of the gasket.
However, these gaskets with hooked material are not fully satisfactory.
Firstly, the packing is made in the form of a semi-worked material which requires a processing of the products, a large stock of the various cardboards employed, inspections upon receipt of the cardboards and inspection of the finished products.
The sealing properties in the heart of the packing, and therefore the gasket including such packing, are also open to criticism.
Further, the mounting of the strengthening elements must be achieved by special techniques, namely, adhesion, an insert combined with seam means the utility of which is reduced to merely the maintenance in position of the insert before mounting the gasket on the engine, spot-welding on the metallic part prior to the mounting of the packing, for example.
Cylinder head gaskets also ensure the sealing of the associated cooling and lubricating fluid circuits, and, in the case of gaskets having hooked material, it is most often necessary to employ a screen printing operation. These gaskets having hooked material must very often undergo additional treatments such as impregnation or varnishing.
The metallic gaskets, more precisely the metallic-elastomeric gaskets, are made in different ways. A first type of such gaskets comprises a plurality of superposed previously cut-out sheets, with seal means on the periphery of the apertures corresponding to the cylinders, and molded elastomeric beadings.
Thus the gas-tight seal is obtained in the known way by metallic means which ensure good resilient reaction quality, zero creep and good thermal behavior.
A liquid-tight seal is achieved by the molded elastomeric beadings.
Theoretically, such a gasket allows a substantial reduction in cylinder block-cylinder head clamping loads, since the pressures of contact are localized solely in the region where a seal is necessary.
However, the difference in the stiffness between the eyelet and the elastomeric beading is such that this assembly is rendered difficult if clamping bearing elements are not judiciously disposed on the perimeter of the gasket.
In this case, the distributions of pressure are very discontinuous, and this results in large concentrations of stresses in the cylinder head. However, at present this design is the only one which allows a reduction in the clamping loads.
The liquid-tight seal is perfect provided that the molding is of high quality indeed, any defect results in leakage since there is no secondary seal, solely the molded beading constituting the barrier.
On the other hand, sealing against gases is excellent since the arrangement employing eyelets is a well-tried method.
Another type of metallic/elastomeric gasket is at present on the market.
This type comprises a stack of sheets coated with a thin coating of elastomer, a few hundredths of a millimeter in thickness, more particularly selected from those of the nitriles family.
In this case, the clamping loads are rather uniformly distributed over the area of the gasket in a way similar to fibrous gaskets.
The gas-tight seal is obtained by a peripheral deformation of the stack of sheets in the region of the openings corresponding to the bores.
In this case, the gas-tight seal is good, since the slight creep of the nitrile elastomer permits achieving a good adaptability of the eyelet.
On the other hand, this type of gasket has a serious drawback. As the elastomer is deposited in a very thin layer, it permits providing rather long leakage passages for liquids, which imparts to this type of gasket a good liquid-tight seal, but does not allow the absorption of high roughness. Further, the joint plane must be perfectly machined with a very good surface state.
Moreover, it is impossible to vary the thickness of the deposited elastomeric layer since these elastomers become brittle in a thick layer, and in any case, the elastomer does not penetrate between the different sheets of the stack. The different sheets are therefore not bonded to one another by the elastomer, which only has a sealing function.
Further, as the sheets are smooth, there is no resilient effect and the adaptability of such a gasket to requirements remains very poor.
It must also be noted that the deposition of nitrile elastomer is delicate and, when employed in a thin layer, it cannot be produced by molding, and this still further limits its adaptability.
Lastly, such a gasket does not easily permit varying the stiffness locally, so that the gasket is perfectly homogeneous and will be clamped outside the eyelet with a pressure which is identical throughout its area, except on the periphery of the ribs in the region of the associated circuits.